The present invention relates to an adhesion imparting layer which is applied to an insulating material serving as the base for printed circuits, particularly a laminated plastic, and which can be used for the additive technique as well as for the subtractive technique of making printed circuits. The adhesion imparting layer is, for example, used to adhesively attach a metal layer, such as copper, to the insulating base material.
It is known to produce adhesion imparting layers for such purposes where the layers contain a rubber to which a resin has been added. In particular, these layers contain acrylonitrile-butadiene rubber (NBR) which is caused to react under heat substantially with phenol formaldehyde resins. Further, suitable resins as well as fillers and rubbers other than NBR may serve to modify the layer.
It is also known to use NBR together with multifunctional modified polyols and multifunctional isocyanates which form a polyurethane in order to obtain an adhesion imparting layer.
The known processes produce problems in some respects. For example, the reaction between the rubber and the phenol resin does not take place at the desired speed, which is consistent with the fact that it is known that the resin vulcanization of rubbers takes a long time. It is possible to accelerate this reaction, principally by the use of accelerating additives, but the possible acceleration of this reaction cannot be used, if, as is often the case, the accelerating additives have a negative effect on the further process steps employed in the additive technique, e.g., poisoning of the baths, or if the accelerating additives adversely affect the electrical properties of the printed circuit.
On the other hand, the process of hardening with phenol resin must not be continued too far since otherwise the subsequent surface roughness effected by means of etching baths will no longer be sufficient, with the consequence of incomplete metallization or poorer adhesion of the metal on the substrate.
In the case of the NBR-polyol-isocyanate compositions which lead to a polyurethane, their stability with respect to the various acid or base treatment baths which are used in subsequent steps of the manufacture of the printed circuit must be considered. Furthermore, there is the danger with some substances from this class that they soften too much during the soldering steps which are performed subsequently in the manufacture of the printed circuit.
Most of all, however, the above-mentioned adhesion imparting layers have substantial drawbacks in those cases where the base material has a completely different cross-linking system which is not compatible with the adhesion imparting layer. Thus, for example, a copper foil provided with an adhesion imparting layer of an acrylonitrile-butadiene (nitrile) rubber-phenol resin often exhibits insufficient adhesion after being pressed together with the components of the base material such as reinforcement layers which have been impregnated with unsaturated polyester resins. This drawback becomes particularly apparent if the adhesion imparting layer and the base material are processed into a pressed laminate in a single process step. The reason for this is probably that in this type of processing, the components of the phenol containing adhesion imparting layer have an inhibiting effect on the cross-linking of the unsaturated polyester resin which contains peroxides.
It is also known to use thermoplastic ABS polymers as an adhesion imparting layer. Such thermoplastic adhesion imparting agents are not suited for the production of printed circuit boards because the adhesion on the conventional base material is usually unsatisfactory and because under increased heat stresses, e.g., during soldering, they soften too much.